1. Field of the Invention
The present invention relates to an oxygen detector adapted to be mounted in exhaust manifolds or pipes forming the exhaust system of an automobile for detecting the residual oxygen content in the exhaust gasses.
2. Description of the Prior Art
Oxygen detectors for the aforementioned purpose are of course known. The detector generally comprises a body enclosing a solid electrolyte oxygen sensor at one end thereof, said sensor generally including a tubular element closed at one end thereof, said tubular element being made of a ceramic or zirconia and covered with very thin platinum layers at its inner and outer surfaces. A flange is usually connected to the body for mounting it at a wall portion of an exhaust manifold or pipe of an engine in a manner such that the sensor end of said body is inserted into the exhaust manifold or pipe through an opening formed in the wall portion of the exhaust manifold or pipe. Furthermore, a cylindrical protective cover is mounted over said sensor for protecting the aforementioned very delicate structure of the solid electrolyte oxygen sensor. The conventional protective cover is generally formed with a plurality of small openings arranged uniformly in its entire cylindrical wall so that the detector may be mounted to the wall portion of an exhaust manifold or pipe in any direction or angle around the central axis of the detector body.
In view of effecting a quick responsive feedback control for the intake system of an engine, it is desirable that the oxygen detector be located as close to the exhaust port of the engine as possible in the exhaust system. Furthermore, since the solid electrolyte oxygen sensor of the aforementioned platinum type can operate in a normal condition when the electrode is heated at or above a predetermined relatively high temperature (generally 400.degree. C.), it is also desirable from this point of view that the oxygen detector be located closer to the exhaust port so that the sensor is heated up to the high temperature by exhaust gasses of a higher temperature within a short period after the starting up of the engine. If the oxygen detector is located very close to the exhaust port, it is naturally exposed to a strong hot stream of exhaust gasses. In this case, if the protective cover mounted over the solid electrolyte oxygen sensor is formed with a lot of small openings arranged uniformly in its entire cylindrical wall as in the conventional detectors, a strong gas flow impingement on the sensor surface will occur in the up-stream side thereof thereby generating hot spots in the platinum layer where it will bulge out and finally there is a danger that the very thin platinum layer is exfoliated.